Since approximately one hundred years ago, projectors have been used to project an image that is disposed on a substrate material and wherein the substrate material is oriented with respect to the projection apparatus by a frame support. A projection beam of light is then directed at the image and the projection beam is modified by the image on the substrate so that the projection beam, which when striking a viewing surface, reproduces the image so that an observer may see the same. Various optical elements are traditionally employed to direct the projection beam.
Early projection devices have been vastly improved in recent times and have evolved from simple projectors to modern day complex electro-mechanical devices having complex operative mechanisms and optical elements. These various projectors include single frame projectors, such as slide projectors, overhead projectors and the like, to motion projectors wherein a plurality of individual frames are consecutively projected at a sufficient frequency to create the illusion of continuous movement of the images projected in all such devices. Regardless of the complexity of projectors several basic elements are employed. These elements include a light source which creates a projection beam, optical elements that guide the projection beam and a frame support structure which interrupts the projection beam when a substrate containing an image is placed thereon.
Most commonly, the substrate material is a transparency, and the projection beam is transmitted through the transparency such that the projection beam thereafter carries with it the information contained in the image that has been placed on the transparency. The image on the transparency is, accordingly, reproduced on the viewing surface so that a viewer may see a reproduction of the image. The reproduction image is usually substantially larger that the original image so that the viewing surface may be simultaneously viewed by a relatively large number of observers.
In early days, projection devices were primarily employed for amusement, and the present day use of projection apparatus for entertainment purposes can not be underestimated. However, projection devices are now widely used in non-entertainment activities such as business and teaching since it allows the projectionist the ability to employ images disposed on a substrate, hereinafter referred to as a transparency, as a visual accompaniment to an audio presentation. Uses of projection apparatus in these activities often are single frame projectors although moving picture projectors are also used. The present invention primarily concerns the single frame projectors generally and, specifically concerns overhead projectors.
An overhead projector is well-known to be a device wherein a light box houses a light source and some associated optical elements and includes a top horizontal transparent stage or light table that defines a frame support. The projection beam is first projectioned vertically upwardly through a transparency, containing an image, which is placed on a light table. The projection beam containing the image from the transparency is then received by additional optical elements, typically supported vertically above the light table a support post, with these second optical elements defining a projection head which directs the projection beam horizontally onto a vertically oriented viewing surface, such as a movie screen or similar area. The projectionist may then discuss the information contained on the image with the observers who are able to see the image on the viewing screen. With this device, the projectionist can typically read from the transparency and can even use a small pointer to point to items on the transparency while the observers may refer to and see the indexed material on the viewing screen.
It is sometimes the case that the projectionist desires to mask a portion of the transparency so that the observers may only view the unmasked portion of the transparency. Unless otherwise equipped, the projectionist may use, for example, a blocking panel such as piece of paper. That portion of the transparency covered by the panel will be obscured from the view of the observers in that the projection beam is blocked. The edge of the panel may further be used as a line of demarcation between the masked and unmasked portion and can serve to highlight certain information contained on the transparency.
Recognizing the disadvantages and awkwardness of utilizing a piece of paper or other material as a masking panel, others in the past have developed devices which operate selectively to mask and unmask portions of the projected image. These masking apparatus have been employed at various locations along the path of the projection beam to interrupt that beam. One such apparatus is shown in U.S. Pat. No. 3,661,449 issued May 9, 1982 to Wright. The Wright Patent describes a shutter assembly which may be employed with an overhead projector with this shutter assembly being supported directly on the light table, intermediate the light table and the projector head, or forwardly of the projector head. In any event, the shutter apparatus taught in the Wright Patent includes a framework having an opening through which the projection beam may pass and a plurality of shutters which are slideably received in the framework. The shutters divide the image area into quadrants which may be selectively masked and unmasked. Where the shutter assembly in this patent is employed directly on the light table, the device includes an upper transparent sheet of glass or plastic which defines an auxillary stage to support the transparency, and the shutter panels then slide in the region between the light table of the projection apparatus and the auxillary stage of the shutter assembly.
U.S. Pat. No. 4,523,823 issued Jun. 18, 1985 to Roop discloses a shutter device for an overhead projector. Here, the projection head of an overhead projector is provided with a flip panel of opaque material which is movable between a raised position allowing passage of the projection beam to a free hanging vertical position that it blocks the projection beam. This device is used primarily to either unblock or block the entire image so that the need to continuously turn the projector on and off to disable the projection beam, is removed.
Other prior art devices utilize different projecting or viewing apparatus other than overhead projectors, are shown in U.S. Pat. No. 3,881,815 issued May 6, 1975 to McGourty, U.S. Pat. No. 4,609,268 issued Sep. 2, 1986 to Crawford and U.S. Pat. No. 4,813,780 issued Mar. 21, 1989 to Solyntjes. In the devices shown in each of these patents, some sort of sliding structure, either to move a substrate or to define a blocking panel, is employed in conjunction with frame support.
Despite the improvements shown in the above described devices, there remains a need for an improved masking device which may be constructed as either original equipment on an overhead projector or as an auxillary device which may retro-fit on existing overhead projectors. In any event, there is a need for a masking device which is more user friendly; for example, there is a need for such a device to permit the projectionist to view an entire transparency while nonetheless blocking portions of the projection beam so that the observers do not see the entire image. Naturally, the usefulness of such masking devices would be enhanced where their construction were made simply and relatively inexpensively.